A power steering device disclosed in Japanese Patent Provisional Publication No. 2003-137117 (hereinafter is referred to as “JP2003-137117”) is generally known as this type of power steering device. The power steering device disclosed in JP2003-137117 is comprised of an output shaft linked to the lower end of a steering shaft, a rack-and-pinion mechanism installed on the lower end of the output shaft for steering of steered road wheels, a hydraulic power cylinder linked to the rack of the rack-and-pinion mechanism, and a motor-driven reversible pump provided for selectively supplying working fluid into the first cylinder chamber arranged as the left-hand half of the power cylinder or into the second cylinder chamber arranged as the right-hand half of the power cylinder. When a normal steering operation is made by means of a steering wheel for left or right turns during vehicle driving, for the purpose of steering assist force application, working fluid (hydraulic pressure or working pressure) is selectively supplied to either one of the first and second hydraulic cylinder chambers by way of normal rotation or reverse rotation of the motor-driven reversible pump. As compared to an electric power steering device that a steering shaft is driven directly by means of an actuator (an electric motor), for the same size and type of electric motor, the hydraulic power cylinder equipped power steering device can produce a comparatively great steering assist force.
However, in recent years, owing to the expansion of power-steering-device equipped vehicles, a greater steering assist force is required. Thus, a further rise in motor power output has come to be required. For the further motor power output rise, Japanese Patent Provisional Publication No. 2003-33077 (hereinafter is referred to as “JP2003-33077”), corresponding to U.S. Pat. No. 6,987,371, teaches the use of a booster circuit that boosts a source voltage of a driving power source for a motor connected to a steering system. JP2003-33077 also teaches a steering controller that controls a source-voltage boost timing based on a rotational speed of the motor. The motor-speed dependent source-voltage boost timing control contributes to a reduction in the frequency of executions of boost operation, thus ensuring the reduced electric power consumption of the car battery.